Cavity-backed slot antennas (CBSA) have been extensively investigated for applications to airborne and satellite communications because they satisfy the requirements of flush mounting, low cost and light weight. Their optimum size scales with the wavelength of the desired radiation frequency which the antenna transmits and/or receives. In order to get the antenna to radiate efficiently, the cavity height is usually designed to be one- or three-quarter wavelengths at the resonator frequency in order not to destroy impedance matching. At low frequencies, such as the VHF and UHF bands, where the radiation wavelength is 1 m or longer, the CBSA can be very large and hard to mount on aircraft. Embodiments of the principles of the present invention described below comprise a reduced-size CBSA that radiates efficiently at low frequencies over a large bandwidth with a tunable operation band.
The prior art teaches that the CBSA cavity height can be reduced through dielectric loading but then the bandwidth and efficiency will also be reduced.
Itoh and Yang (U.S. Pat. No. 6,518,930) have disclosed a CB SA loaded with a passive Artificial Magnetic Conductor (AMC) structure. The AMC transforms the cavity ground plane into an electrically open surface, and allows the CBSA to operate at lower frequencies without an excessively deep cavity. However, the measured bandwidth of the antenna is very narrow because they use the passive AMC structure to load the CBSA.